Golf ball

ABSTRACT

In a golf ball having a core and a cover of one or a plurality of layers that encases the core, the core or at least one layer of the cover is formed of a resin composition which contains from 5 to 250 parts by weight of a mechanoluminescent material that receives mechanical energy and luminesces per 100 parts by weight of a base material which is a rubber material or a resin material. This golf ball has an excellent visibility in that, even under dark conditions, the ball impact site can be clearly confirmed visually at the time of impact. Moreover, the ball is able to maintain a good scuff resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2016-123266 filed in Japan on Jun. 22, 2016, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates generally to golf balls. More particularly, the invention relates to a golf ball which is suitable for use in night golf and for use at indoor golf ranges and golf practice ranges.

BACKGROUND ART

A number of disclosures have hitherto been made relating to golf ball materials that contain a luminescent material or a fluorescent material. For example, JP-Y 3015308 describes a golf ball in which a phosphorescent paint has been applied to the surface of the ball cover. JP-A H04-122372 teaches art in which a luminous paint is applied to the skin of a golf ball. Luminous paints mentioned in this art include phosphorescent substances such as acrylics that collect sunlight or light from fluorescent lamps and luminesce when the surroundings darken.

However, the golf balls in the foregoing art all use phosphorescent paint or luminous paint on the surface of the golf ball, that is, as a paint film, and so this art requires the formation of a thicker paint film than in the past. Hence, there is a concern that problems such as crazing of the paint film or a decrease in the distance of the ball may arise.

Also. JP-A S61-176369 discloses a golf ball for use in night golf that has a surface layer containing specific amounts of phosphorescent pigment and glass beads with respect to the base resin. Mention is made here of the use of, for example, ZnS—Cu as the phosphorescent pigment.

However, in this golf ball that contains a phosphorescent pigment and glass beads in the cover layer, when the content of phosphorescent paint and glass beads is high, the ball durability and scuff resistance tend to worsen.

In addition, JP-A 2003-339915 teaches art wherein a paint containing a mechanoluminescent material which emits light upon receiving mechanical energy at the time of ball impact is applied to the outer skin of a golf ball. However, in this art, a paint film is formed using paint that contains a large amount of mechanoluminescent material, and one concern is that the paint film may become brittle, allowing scuff marks to readily form on the ball upon impact.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a golf ball which has an excellent visibility in that, even under dark conditions, the ball impact site can be clearly confirmed visually at the time of impact and which, moreover, is able to maintain a good scuff resistance.

As a result of extensive investigations, we have discovered that, in a golf ball having a core and a cover of a single layer or a plurality of layers that encases the core, by forming the core or at least one layer of the cover of a resin composition which includes from 5 to 200 parts by weight of a mechanoluminescent material per 100 parts by weight of a rubber material or resin material serving as the base material, the ball has an excellent visibility in that, even under dark conditions, the ball impact site can be clearly confirmed visually at the time of impact, and the ball moreover is able to maintain a good scuff resistance. Furthermore, at indoor and practice ranges, golf balls are often repeatedly used. Such golf balls that have been used over a long period of time, even if they do not crack, are often observed to have a roughened surface due to scuffing on impact. Even if it is possible to hit a golf ball that is damaged in this way, such damage may cause the ball to behave abnormally during flight. Therefore, the ability to maintain a good scuff resistance, which is both an object and an advantageous effect of this invention, is an important golf ball property that differs from the durability of the ball to cracking.

Accordingly, the invention provides a golf ball having a core and a cover of one or a plurality of layers that encases the core, wherein the core or at least one layer of the cover is formed of a resin composition which contains from 5 to 250 parts by weight of a mechanoluminescent material that receives mechanical energy and luminesces per 100 parts by weight of a base material which is a rubber material or a resin material.

The mechanoluminescent material is preferably made of strontium aluminate crystals doped with primarily at least one type of rare-earth ion.

The mechanoluminescent material-containing resin material is preferably an ionomer resin.

In one preferred embodiment of the golf ball of the invention, an outermost layer of the cover is formed of a mechanoluminescent material-containing resin composition, the content of the mechanoluminescent material being from 15 to 100 parts by weight per 100 to parts by weight of the resin material.

In another preferred embodiment, the cover has a plurality of layers, and an inner cover layer adjoining an outermost layer of the cover is formed of a mechanoluminescent material-containing resin composition. The content of the mechanoluminescent material included in the inner cover layer is preferably from 20 to 250 parts by weight per 100 parts by weight of the resin material.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The golf ball of the invention, by including a special mechanoluminescent material in the core or at least one layer of the cover, enables the ball impact site to be clearly confirmed visually at the time of impact--even under dark conditions, and moreover enables the ball to obtain an excellent scuff resistance.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from e following detailed description.

The golf ball of the invention has a core and a cover of one or a plurality of layers that encases the core. Various types of cores, such as a solid core or a wound core, may be used as the golf ball core in this invention.

When the core is a solid core, it can be formed using a known rubber composition. The base rubber is exemplified by polybutadiene, with the use of cis-1,4-polybutadiene having a cis structure content of at least 40% as the chief component being recommended in particular, in addition, natural rubber, polyisoprene rubber, styrene-butadiene rubber or the like may also be included within this base rubber.

The metal salts or ester compounds of unsaturated fatty acids, such as methacrylic acid or acrylic acid, may be included as co-crosslinking agents in the rubber composition. Such metal salts are exemplified by the zinc salts, magnesium salts, and calcium salts, and such ester compounds are exemplified by trimethylolpropane trimethacrylate. The content of these co-crosslinking agents per 100 parts by weight of the base rubber is generally at least 10 parts by weight, and preferably at least 15 parts by weight, with the upper limit being 50 parts by weight or less, and preferably 40 parts by weight or less.

An organic peroxide may be included in the rubber composition. Illustrative examples include 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, dicumyl peroxide, di(t-butylperoxy)-m-diisopropylbenzene and 2,5-dimethyl-2,5-di-t-butylperoxyhexane. Illustrative examples of such commercial products include Percumyl D (NOF Corporation) and Perhexa 3M-40 (NOF Corporation). The content of these organic peroxides per 100 parts by weight of the base rubber is generally at least 0.1 part by weight, and preferably at least 0.5 part by weight, with the upper limit being 5 parts by weight or less, and preferably 2 parts by weight or less.

Various types of additives may also be included, as needed, in the above composition. For example, additives such as sulfur, antioxidants, zinc oxide, barium sulfate, organosulfur compounds such as the zinc salt of pentachlorothiophenol, and zinc stearate may be included. The contents of these additives are suitably adjusted in accordance with various purposes, and are not particularly limited.

The diameter of the core is suitably selected according to the intended ball construction, and is not particularly limited. For example, when a two-piece solid golf ball or a three-piece solid golf ball is to be produced, the core diameter is preferably at least 32.0 mm, and more preferably at least 33.0 mm, with the upper limit being preferably 40.5 mm or less, and more preferably 39.5 mm or less.

The core has a deflection (mm) when compressed under a final load of 1,275 N from an initial load of 98 N which, although not particularly limited, is preferably from 2.0 to 5.0 mm, more preferably from 2.2 to 4.5 mm, and even more preferably from 2.4 to 4.0 mm. When this value is too small, that is, when the core is too hard, the feel of the ball when struck with a driver may be hard and the contact time between the club and the ball may be too short, resulting in poor controllability. On the other hand, when the deflection (deformation) is too large, the feel of the ball when struck with a driver may be too soft, or the durability of the ball to cracking on repeated impact may worsen.

The core may be produced by a known method. For example, to obtain a solid core from a core-forming rubber composition, use can be suitably made of a method in which the composition is intensively mixed using a conventional mixing apparatus (such as a Banbury mixer, kneader or roll mill), and the resulting compound is compression molded in a core mold. Alternatively, a wound core may be obtained by a commonly used method that is known to the art.

Next, in this invention, the cover encasing the core may be composed of a single layer or a plurality of layers. A thermoplastic resin or a thermoset resin may be used as the base material for each layer of the cover, with the use of a thermoplastic resin or a thermoplastic elastomer being especially preferred. Examples of thermoplastic resins include ionomer resins. Commercial products that may be used include those of the trade names Himilan (from DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn (E.I. DuPont de Nemours and Co.) and Iotek (Exxon). Examples of thermoplastic elastomers include polyester, polyamide, polyurethane, olefin and styrene-based thermoplastic elastomers. Commercial products that may be used include those of the trade names Hytrel (from DuPont-Toray Co., Ltd.), Pelprene (Toyobo Co., Ltd.), Pebax (Toray Industries, Inc.), Pandex (DIC Covestro Polymer Ltd.), Santoprene (Monsanto Chemical Co.), Tuftec(Asap Chemical Industry Co., Ltd.) and Dynaron (JSR Corporation). An ionomer resin or a thermoplastic polyurethane elastomer is preferably used as the thermoplastic resin or thermoplastic elastomer.

In this invention, the core or at least one layer of the cover contains a mechanoluminescent material that receives mechanical energy and luminesces.

Preferred use can be made of any of the mechanoluminescent materials described in JP-A 2003-339915. These mechanoluminescent materials are either mechanoluminescent materials obtained by doping an inorganic matrix material with luminescent centers consisting of at least one type of rare-earth or transition metal that emits light when electrons excited by mechanical energy return to the ground state, or mechanoluminescent materials having lattice detects that emit light when electrons excited by mechanical energy return to the ground state.

Examples of these inorganic matrix materials include oxides, sulfides, carbides and. nitrides having a melilite structure, an FeS₂ structure, a wurtzite structure, a spinet structure, a corundum structure or a β-alumina structure.

The luminescent centers doped in the inorganic matrix material are preferably of one or more type from among rare-earth ions such as Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dv, Ho, Er, Tm, Yb and Lu, and transition metal ions such as Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ta and W.

The mechanoluminescent material used in this invention is preferably a material obtained by doping crystals of strontium aluminate (SAO), which is a double oxide of aluminum and strontium, with primarily rare-earth ions, and preferably europium (Eu) ions.

The mechanoluminescent material used in this invention is generally in the form of a powder. The powder has a primary particle size, as measured by the laser diffraction/scattering method, of preferably from 1 to 5 μm, and more preferably from 2 to 3 μm. When the particle size is too large, strong luminescence is obtained, but the durability and scuff resistance of the ball may worsen. On the other hand, when the particle size is too small, the luminescence may weaken.

It is recommended that the core or cover which includes this mechanoluminescent material have a material hardness, expressed in terms of Shore D hardness, which is preferably at least 35, more preferably at least 40, and even more preferably at least 45. The upper limit is preferably 90 or less, more preferably 85 or less, and even more preferably 80 or less. When the cover hardness is too high, the durability to cracking on repeated impact may decrease or the feel of the ball at impact may become too hard. When the cover hardness is too low, the rebound may decrease and the spin rate may rise, resulting in a poor distance.

The viscoelasticity (tan δ) of the core or cover material containing the mechanoluminescent material is preferably 0.5 or less, more preferably 0.4 or less, and even more preferably 0.3 or less. When this is too large, the rebound may be low and a good distance may not be obtained. This viscoelasticity is defined as the ratio between storage modulus and loss modulus in dynamic viscoelasticity measurement at a temperature of 23° C., an oscillation frequency of 15 Hz and 1.0% strain.

In order to be able to exhibit the desired effects of the invention, the content of mechanoluminescent material used in this invention is set to from 5 to 250 parts by weight, and preferably from 10 to 200 parts by weight, per 100 parts by weight of the base rubber or base resin in the core or at least one layer of the cover. When this content is low, a ball impact site luminescing effect is not sufficiently obtained. On the other hand, when this content is high, the durability and scuff resistance of the ball worsen.

When a mechanoluminescent material is included in the outermost layer of the cover, the content of the mechanoluminescent material is preferably from 15 to 100 parts by weight per 100 parts by weight of the resin material in the outermost layer. In the golf ball of the invention, when the cover has a plurality of layers, the mechanoluminescent material may be included in an inner cover layer adjoining an outermost layer of the cover. In this case, adjusting the content of mechanoluminescent material within the range to preferably 20 to 250 parts by weight, and more preferably 40 to 200 parts by weight, per 100 parts by weight of the resin material in the inner cover layer, that is, so as to be higher than when the mechanoluminescent material is included in the outermost layer, is desirable from the standpoint of the luminescing effect and visibility under dark conditions.

The resin composition of the outermost layer has a percent haze of preferably 80 or less, and more preferably 70 or less. When the percent haze is high, the outermost layer loses a sense of transparency and it may not be possible to fully exhibit the luminescing effect by the mechanoluminescent material included in the inner cover layer. Here, “percent haze” is the value computed as shown below from total light transmittance and diffuse transmittance. Reference may be made to JIS K7136 (2000) for details.

Percent haze=(diffuse transmittance T _(d))/(total light transmittance T ₁)×100 (%)

At a larger haze value, clouding is stronger. Conversely, when this value is smaller, the diffusion of light is lower and the transparency is higher. The total light transmittance and diffuse transmittance mentioned above refer to the light transmission ratio and diffusion ratio when light from a light source is made to pass through a test specimen of plastic (resin composition for outermost layer), and are values in conformity with JIS K7105 (1981).

It is desirable to use a transparent or translucent resin material as the resin material for satisfying the above percent haze value. For example, use can be made of the above-mentioned base resins, such as ionomer resins, polyurethane elastomers, polyester elastomers, polyamide elastomers and polyolefin elastomers. Also, various elastomers and additives, effect pigments, colorants and the like may be added to these resin materials within ranges that do not detract from the transparency.

Various types of additives such as UV absorbers, antioxidants, metallic soaps, and pigments and inorganic tillers other than those mentioned above may be suitably included in the base resins for the respective layers of the cover, provided that these are included in to suitable amounts within ranges that do not detract from the objects and advantageous effects of the invention.

The thicknesses of the individual cover layers are not particularly limited, although it is desirable for these layers to be formed to a thickness of preferably at least 0.3 mm, more preferably at least 0.5 mm, and even more preferably at least 0.7 mm, with the upper limit being preferably 3.0 mm or less, more preferably 2.7 mm or less, and even more preferably 2.4 mm or less. When the individual cover layers are too thin, durability to cracking under repeated impact may worsen. On the other hand, when the individual cover layers are too thick, the feel of the ball at impact may worsen.

When the cover is formed as two or more layers, the thickness of the inner cover layer may be set in the same range as described above, but it is desirable for the thickness of the outermost layer of the cover to have an upper limit of preferably 1.5 mm or less, and more preferably 1.2 mm or less.

A known method such as injection molding or compression molding may be used to obtain a golf ball in which the above cover encases the core. For example, when injection molding is carried out, the golf ball may be produced by setting a prefabricated solid core within a mold and, in the usual manner, introducing a cover material into the mold. By way of illustration, a three-piece solid golf ball having a core and a cover of two layers can be produced by carrying out this injection molding process twice, once for each layer of the cover. In this case, because numerous dimples are typically formed on the surface of the outermost layer of the cover, it is preferable to use, during injection molding of the outermost cover layer, a mold having numerous dimple-forming projections formed in the cavity thereof.

In the practice of this invention, the golf ball may be rendered into a finished product by carrying out painting and the like in accordance with ordinary known methods. The paint used is not particularly limited, and may be a clear coating that helps the cover luminesce effectively, or may include an effect pigment such as a pearlescent pigment. The thickness of the golf ball paint film is preferably within a range of 5 to 25 μm.

It is recommended that the golf ball of the invention have a deflection (deformation) when compressed under a final load of 1.275 N (130 kg) from an initial load of 98 N (10 kgf) which is generally at least 2.0 mm, preferably at least 2.3 mm, and more preferably at least 2.5 mm, with the upper limit being typically 5.0 mm or less, preferably to 4.0 mm or less, and more preferably 3.7 mm or less. When the ball deflection is smaller than this range, the feel of the ball at impact may be hard and the contact time between the ball and the club at the time of impact may be too short, resulting in poor controllability. On the other hand, when the ball deflection is larger than this range, the feel may be too soft and the durability to cracking on repeated impact may worsen.

For competitive play, the golf ball of the invention can be made to conform with the Rules of Golf. Specifically, the ball may be formed to a diameter of not less than 42.67 nun and a weight of not more than 45.93 g. The upper limit in the diameter is preferably 44.0 mm or less, more preferably 43.5 mm or less, and most preferably 43.0 mm or less. The lower limit in the weight is preferably 44.5 g or more, more preferably 45.0 g or more, even more preferably 45.1 g or more, and still more preferably 45.2 g or more.

EXAMPLE

Working Examples and Comparative Examples are provided below to illustrate the invention, and are not intended to limit the scope thereof.

Working Examples 1 to 7, Comparative Examples 1 to 3

Solid cores were produced by using the rubber composition shown in Table 1 below, which composition was common to all the Examples, and vulcanizing for 15 minutes at 155° C.

TABLE 1 Rubber composition (pbw) A Polybutadiene 100 Zinc oxide 4.00 Barium sulfate 1.8 Calcium carbonate 22.0 Antioxidant 0.1 Zinc acrylate 26 Organic Peroxide (1) 0.6 Organic Peroxide (2) 0.6

Details on the above solid core materials are given below.

-   -   Polybutadiene: Available under the trade name “BR01” from JSR         Corporation     -   Zinc oxide: Available from Sakai Chemical Co., Ltd.     -   Barium sulfate: Available from Sakai Chemical Co., Ltd.     -   Calcium carbonate: Available from Shiraishi Calcium Kaisha, Ltd.     -   Antioxidant: Mono-(or di- or tri-)(α-methylbenzyl)phenol,         available under the trade name “Nocrac SP-N” from Ouchi Shinko         Chemical Industry Co., Ltd.     -   Zinc acrylate: Available from Nippon Shokubai Co., Ltd.     -   Organic Peroxide (1): Dicumyl peroxide, available under the         trade name “Percumyl D” from NOF Corporation     -   Organic Peroxide (2): A mixture of         1,1-di(tert-butylperoxy)cyclohexane and silica, available under         the trade name “Perhexa C-40” from NOF Corporation

Next, in Working Examples 1 to 6 and Comparative Examples 1 to 3. cover materials formulated as shown in Table 2 below were mixed in a kneading-type twin-screw extruder, giving pelletized cover materials, following which these cover materials were injected into a mold in which the solid core had been placed, thereby producing two-piece solid golf balls. In Working Example 7 alone, separate cover materials for the inner cover layer and the outer cover layer were successively injection-molded over the core, thus producing a three-piece solid golf ball. Dimples common to all the Examples were formed on the surface of the cover at this time.

TABLE 2 Resin material ingredients (pbw) a b c d e f g h i j Himilan 50 50 50 50 50 50 50 50 50 50 1605 Himilan 50 50 50 50 50 50 50 50 50 50 1706 Mechanoluminescent 5 10 20 40 100 150 230 0 2 70 material Total 105 110 120 140 200 250 330 100 102 170 Shore D 65.7 66.3 67.2 68.1 71.8 75.1 79.0 64.4 65.3 69.5 hardness

Details on the materials mentioned in Table 2 are given below.

-   -   Himilan 1605: A sodium-neutralized ionomer from DuPont-Mitsui         Polychemicals Co., Ltd.     -   Himilan 1706: A zinc-neutralized ionomer from DuPont-Mitsui         Polychemicals Co., Ltd.     -   Mechanoluminescent material:         -   The mechanoluminescent powder ML-032 (strontium aluminate             (SAO) crystals doped primarily with the rare-earth europium)             from Sakai Chemical Co., Ltd.

Shore D Hardness of Cover Material

The cover material (resin composition) was molded into a sheet having a thickness of 2 mm and left to stand for at least two weeks, following which the Shore D hardness was measured in accordance with ASTM D2240-95.

The surfaces of the golf balls obtained above were coated to a thickness of 16 μm with the coating compositions in Table 3 below, thereby producing golf balls as finished products.

TABLE 3 Coating formulation (pbw) l m Base resin of clear coat (two-part urethane mixture) 66.7 66.7 Hardener of clear coat (two-part urethane mixture) 33.3 33.3 Mechanoluminescent material 0 100 Total 100.0 200.0

-   -   Base resin: from Asia. Industry Co., Ltd.     -   Hardener: from Asia Industry Co., Ltd.     -   Mechanoluminescent material:         -   Same as mechanoluminescent material used in cover material

The ball diameter, deflection, luminescent state and scuff resistance for each of the golf balls obtained above were measured or evaluated by the following methods. The results are shown in Table 4.

Golf Ball Diameter

The diameters at five random dimple-free areas on the surface of a ball were measured at a temperature of 23.9+1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for five measured balls was determined.

Deflection of Golf Ball

The golf ball was placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The amount of deflection here refers in each case to the measured value obtained after holding the ball isothermally at 23.9° C.

Luminescent State

Three balls of each type were hit with a driver (W#1) at a head speed of 40 m/s, the impact site of each ball was visually observed and rated based on the following 5-point scale, and the average score was calculated for each type of ball.

5: Luminescence can be even more clearly confirmed by naked eye.

4: Luminescence can be clearly confirmed by naked eve.

3: Luminescence can be confirmed by naked eye.

2: Slight luminescence can be confirmed by naked eye.

1: Confirmation of luminescence by naked eye is difficult.

Scuff Resistance

The golf balls were held isothermally at 23° C. and five balls of each type were hit at a head speed of 33 m/s using as the club a pitching wedge mounted on a swing robot machine. The damage to the ball from the impact was visually rated based on the following 5-point scale, and the average score was calculated for each type of ball.

5: No damage or substantially no damage.

4: Damage is apparent but so slight as to be of substantially no concern.

3: Surface is slightly frayed.

2: Some fraying of surface or loss of dimples.

1: Dimples completely obliterated in places.

TABLE 4 Comparative Working Comp. Working Example Example Ex. Ex. 1 2 3 4 5 6 1 2 7 3 Core Core A A A A A A A A A A composition Diameter (mm) 39.30 39.30 39.30 39.30 39.30 39.30 39.30 39.30 38.55 39.30 Cover Structure 1 1 1 1 1 1 1 1 2 1 layer layer layer layer layer layer layer layer layers layer Inner cover none e none layer material Thickness (mm) 1.1 Outer cover a b c d e f i g h h layer material Thickness (mm) 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.0 1.7 Coating Type 1 1 1 1 1 1 1 1 1 m Product Diameter (mm) 42.67 42.73 42.72 42.72 42.69 42.69 42.70 42.69 42.71 42.69 Deflection (mm) 2.8 2.8 2.7 2.7 2.6 2.5 2.8 2.4 2.7 2.8 Evalution Luminescent 2 2 3 3 4 5 1 5 3 3 State Scuff resistance 5 5 4 4 3 2 3 1 4 1

Japanese Patent Application No. 2016-123266 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A golf ball comprising a core and a cover of one or a plurality of layers that encases the core, wherein the core or at least one layer of the cover is formed of a resin composition which contains from 5 to 250 parts by weight of a mechanoluminescent material that receives mechanical energy and luminesces per 100 parts by weight of a base material which is a rubber material or a resin material.
 2. The golf ball of claim 1, wherein the mechanoluminescent material is a material comprising strontium aluminate crystals doped with primarily at least one type of rare-earth ion.
 3. The golf ball of claim 1, wherein the mechanoluminescent material-containing resin material is an ionomer resin.
 4. The golf ball of claim 1, wherein an outermost layer of the cover is formed of a mechanoluminescent material-containing resin composition, the content of the mechanoluminescent material being from 15 to 100 parts by weight per 100 parts by weight of the resin material.
 5. The golf ball of claim 1 wherein the cover has a plurality of layers, and an inner cover layer adjoining an outermost layer of the cover is formed of a mechanoluminescent material-containing resin composition.
 6. The golf ball of claim 5, wherein the content of the mechanoluminescent material included in the inner cover layer is from 20 to 250 parts by weight per 100 parts by weight of the resin material. 